W. van Ieperen

Reconsideration of the use of deionized water as vase water in postharvest experiments on cut flowers

Abstract The relevance of deionized water as a control treatment in vase life experiments and the effects of major tap water components on cut flower water balance were investigated. Chrysanthemum ( Dendranthema x grandiflorum Tzvelev cv. Cassa) was used in all experiments. Deionized water gave a sharp decrease in fresh weight of the cut flowers after 1–3 days. This decrease was absent in tap water. After 4 days in deionized water, hydraulic resistance in the basal part of the stem was ∼50 times the value of fresh cut flowers and seven times the value in tap water. Change in fresh weight during vase life in a solution containing combinations of CaCl 2 , NaHCO 3 and Cu 2+ at concentrations commonly present in tap water was similar to that in tap water. However, none of the minerals tested by themselves gave fresh weight results similar to those from using tap water. In the combined solution, hydraulic resistance in the basal part of the stem after 4 days was comparable to that in tap water. A minimal amount of Cu 2+ (>0.30 mg·l −1 ) enhanced fresh weight, probably by reducing bacterial growth in the cut open vessels. Calcium chloride (>0.7 mM) delayed the increase in hydraulic resistance in the stem (not including the basal 3 cm) compared to deionized water, and at a high concentration (10.7 mM), substantially decreased the transpiration rate. Sodium bicarbonate (1.5 mM) neither affected hydraulic resistance nor transpiration rate, but positively influenced fresh weight change during vase life when combined with CuSO 4 and as compared to deionized water. Results strongly question the appropriateness of deionized water as a control solution in vase life experiments.

Embolism repair in cut flower stems: a physical approach

Abstract The role of xylem anatomical properties on air embolism removal and xylem hydraulic conductance recovery from cut flower stems during the first hours of vase life was studied from a physical point of view. A model based on physical processes was developed and tested using chrysanthemum ( Dendranthema × grandiflorum Tzvelev) cut flowers. The model predicts that the repair process takes place in two major phases. During the first few seconds after replacing in water, initially air-filled vessels at the cut surface partly refill with water. Consequently, reconnections are established between the vase water and the non-cut water-filled xylem vessels just above the cut surface and hydraulic conductance is partly recovered. During the following hours, air partly or completely dissolves into the surrounding water in the stem and hydraulic conductance recovery gradually takes place. The results of the model agreed well with dynamic measurements of hydraulic conductance recovery on chrysanthemum stem segments after aspiration of air. Visual detection of air emboli by cryo-scanning electron microscopy showed that after 1.5 h of repair, air was only present in large-diameter vessels at a position relatively distant from the cut surface of the stem. According to the model, hydraulic conductance repair occurs more readily in stems with smaller diameter vessels. Model calculations and experiments showed that the height of water in the vase influences recovery of water uptake more in stems with large-diameter vessels than in stems with small-diameter vessels. It is concluded that the anatomical structure of the xylem plays an important role in the rehydration capability of cut flowers.

Impact of light on leaf initiation: a matter of photosynthate availability in the apical bud?

Radiation substantially affects leaf initiation rate (LIR), a key variable for plant growth, by influencing the heat budget and therefore the temperature of the shoot apical meristem. The photosynthetically active component of solar radiation (photosynthetic photon flux density; PPFD) is critical for plant growth and when at shade to moderate levels may also influence LIR via limited photosynthate availability. Cucumber and tomato plants were subjected to different PPFDs (2.5-13.2molm-2 day-1) and then LIR, carbohydrate content and diel net CO2 uptake of the apical bud were quantified. LIR showed saturating response to increasing PPFD in both species. In this PPFD range, LIR was reduced by 20% in cucumber and by 40% in tomato plants. Carbohydrate content and dark respiration were substantially reduced at low PPFD. LIR may be considered as an adaptive trait of plants to low light levels, which is likely to be determined by the local photosynthate availability. In tomato and cucumber plants, LIR can be markedly reduced at low PPFD in plant production systems at high latitudes, suggesting that models solely based on thermal time may not precisely predict LIR at low PPFD.

The influence of light intensity and leaf age on the photosynthetic capacity of leaves within a tomato canopy

Summary In dense crop stands, the decrease in leaf photosynthetic capacity (Amax) is paralleled by a decrease in photosynthetically active photon flux density (PPFD) and an increase in leaf age. In greenhouse horticulture, assimilation lighting is traditionally applied from above the canopy. Recently a new lighting technique has been developed in which assimilation lighting is applied within the canopy: intracanopy lighting. This development raises the question whether the decrease in the Amax of lower, thus older and shaded, leaves in a crop is solely due to the lower PPFD, or also partly due to ageing of these leaves. We investigated whether leaf ageing influenced changes in the Amax of tomato leaves during their usual life-span during cultivation in commercial crop systems (i.e., up to 70 d). To uncouple leaf age from the PPFD, tomato plants were grown horizontally, so that the PPFD was similar for all leaves. To investigate the effect of PPFD during leaf development (PPFDld), Amax-leaf age profiles were determined for the leaves of plants grown under conditions with distinctly different natural patterns of PPFD (i.e., Winter, early Spring, and late Spring). In addition, in half of the plants used per experiment, all fully-developed leaves were shaded to 25% of the normal PPFD in the greenhouse using a neutral density filter. Photosynthetic capacity and chlorophyll contents were higher in late Spring than in Winter, but were hardly affected by leaf age. In early Spring, the Amax and chlorophyll contents were higher in younger leaves than in older leaves. To a large extent, this was due to the differences in PPFDld, and hardly due to leaf ageing. Shading fully-developed, mature leaves dramatically decreased their Amax and chlorophyll contents within a few days. We conclude that, during the normal 70 d life-span of tomato leaves in commercial cultivation, the decrease in PPFD within the canopy, and not leaf-ageing, is the most important factor causing changes in Amax with canopy depth.

Differential effects of temperature on stem length and xylem vessel length distribution in Zinnia elegans.

Summary Water transport in vascular plants depends on the hydraulic conductance of the xylem system, which is dependent on the anatomical properties, number, diameter, and length of the xylem vessels. The ability to transport water through their stems influences not only the growth of many horticultural crops, but also the post-harvest quality of cut flowers. In this study, we investigated the effects of different average daily temperatures (ADT) and the difference between day-time (DT) and night-time (NT) temperature (DIF) on stem size, the length of xylem vessels within the stem, and the length of individual vessel elements within a vessel, in Zinnia elegans. Two Z. elegans cultivars, ‘Envy’ and ‘Purple Prince’, were grown in climate chambers under all nine combinations of three DT and three NT temperatures (viz. 17ºC, 21ºC, or 25ºC). An increase in ADT was positively correlated with the lengths of the stems, internodes, and xylem vessels in both cultivars. However, the lengths of the xylem vessels were influenced more strongly than the lengths of the stems. Increasing the ADT from 17ºC to 25ºC increased stem lengths by approx. 15%, but more than doubled the lengths of the xylem vessels. The increase in xylem vessel lengths was only partly (< 10%) due to an increase in the lengths of individual vessel members, which implies that temperature (ADT) had a greater influence on the number of fused vessel elements per xylem vessel. A negative DIF (i.e., lower DT than NT temperatures) decreased stem lengths and a positive DIF increased stem lengths. DIF had no effect on xylem vessel length, probably because, other than in stem length, xylem vessel length was positively correlated with NT temperature.